The proposed longitudinal study (i.e, the Longitudinal Couples Study or LCS) will provide important and novel data about relationship dynamics and sexual risk taking behavior by tracking change over time. It builds on the "Gay Couples Study" (GCS) by following a cross-sectional sample of N = 450 gay male couples longitudinally for 3 years in 6-month intervals. The proposed study will focus on the interplay of changing relationship dynamics and changes in sexual risk over time. Specifically, the aims are to: 1. Examine longitudinally the role of within and between couple differences in relationship dynamics (i.e., intimacy, communication, commitment, relationship quality, trust, agreement quality, and perception of risk) n predicting sexual risk-taking among couples. 2. Examine longitudinally the influence each partners'level of intimacy, communication, commitment, relationship quality, trust, agreement quality, and perception of risk, has on his own and his partner's sexual risk-taking behavior. 3. Examine longitudinally differences in sexual risk-taking and relationship dynamics associated with couple serostatus. 4. Examine the timing and relationship circumstances associated with broken agreements. 5. Describe the context of changing relationship dynamics and how they influence sexual risk over time. Quantitative and qualitative methods will be used to more thoroughly understand relationship dynamics associated with sexual risk over time, to closely track the context of change, how change occurs within the same relationship and how these changes influence sexual risk among couples, over time. The proposed study addresses a gap in HIV prevention that is long overdue, given there are no longitudinal studies examining HIV risk among couples. Findings from this study will be used to develop a future HIV prevention intervention trial uniquely tailored to gay couples.